All land is taken by conquest. That is the nature of our species.
The Europeans took America from the native tribes who held dominion at the time, but those tribes had taken it by force from other tribes who held it previously. And on and on. It is a fundamental fact of human history.
Many "legitimate" countries in the Middle East are made-up countries, carved out of the Ottoman Empire - which was defeated in battle. Are any of these countries "legitimate"? Do the Turks have a claim on all of them because they ruled those lands previously? No. It's a dead issue.
By this point, all of Greater Israel, including Gaza and the West Bank are part of Israel, by conquest. There is only one reason why the conflict between the two societies continues: the emphatic, overtly-professed, neurotic hatred of the Arab population for JEWS (not "Israelis"). Were they to lay down their arms and quell their hatred, they could live peacefully and prosperously in the most successful country in the region. But it cannot be forgotten that Israel is a JEWISH state, and Israel could not be expected to permit itself to be self-annihilated by breeding Arabs stuffing future ballot boxes in a fully-democratic State. Protections would have to be built into the government structure. But unlike Christians in Islamic States, Muslims could live peacefully, and prosper in a free Israel.